1. Field of the Invention
This invention relates to a projection type television apparatus using a green cathode ray tube having an improved color reproduction characteristic.
2. Description of the Related Art
FIG. 8(a), (b) of the accompanying drawings are diagrams showing a structure of a projection lens unit for a cathode ray tube, equipped with a multilayered interference filter is employed in a conventional projection type television apparatus. As shown in FIG. 8(a), (b) the projection television apparatus comprises a cathode ray tube 1 and a series of projection lenses 2 located in front of the cathode ray tube 1. A reference numeral 3 designates an incident pupil position of the series of projection lenses 2; 4, a display face plate pane; 5, a multilayered interference filter; 6, a fluorescent material; and 7, a vacuum evaporation aluminum film.
The operation of this conventional projection television apparatus will now be described.
When electron beams are incident on a fluorescent material 6 coated on the cathode ray tube 1, the fluorescent material 6 shows a characteristic curve 9 of a luminous spectrum of a multilayered interference coating with respect to transmittivity shown in FIG. 5. In view of the life and brightness, any existing cathode ray tube to be used in a projection television apparatus has inevitably such a spectrum. As shown in FIG. 5, the green fluorescent material 6 includes a spectrum blue-side and red-side spectra in addition to an essential green spectral wavelength 545 nm. Therefore, as shown in a chromaticity diagram of FIG. 6, only the chromaticity value at a point G.sub.0 16 remote from the standard green chromaticity point G13 can be obtained. Consequently, color reproduction is possible only within a triangular range 19 indicated by broken lines.
To this end, it is a known practice to place between the face plate pane 4 and the fluorescent material 6 a multilayered interference filter 5 composed of a plurality of alternately superimposed layers of high and low refractive index materials, and then to determine the transmittivity characteristic of this multilayered interference filter as indicated by the broken lines 11 shown in FIG. 5. Thus, unnecessary light spectrum having peak values around 600 nm of the light spectrum emitted from the green fluorescent material 6 is reflected. Therefore it is possible to improve the color reproduction up to the chromaticity point G.sub.1 17 shown in FIG. 6. However, since the transmittivity characteristic of the interference filter 5 is determined as indicated by the broken lines 11 of FIG. 5, it is impossible to reflect unnecessary light spectrum having a peak at a wavelength of a slightly less than 500 nm. As a result, color reproduction is yet possible in the triangular range 20 indicated by the dash-and-one-dot lines in FIG. 6.
Since the image on the cathode ray tube 1 is disposed at an angle of .theta..sub.1 with respect to the incident pupil 3 which is at the center of the projection lens series 2 as shown in FIG. 8(a), the difference l in the optical path between the layers of the multilayered interference filter 5 can be expressed by the following equation: EQU l=2d cos .theta..sub.1 (d: layer thickness)
Consequently, the transmittivity characteristic of, in the direction of angle .theta..sub.1, the multilayered interference filter 5 will be as indicated by the dash-and-one-dot lines 12 in FIG. 5. This causes even the necessary green light spectrum to be reflected, thus impairing the color reproduction range.
Prior to this application, the applicant herein proposed a projection type television having the color reproduction and the purity of green highly improved by interposing the multilayered interference coating layer 23 having a high reflective index to light having a wavelength of 520 nm or less as shown in FIG. 9(a), (b) in the green projection lens series located in front of the cathode ray tube.
As described above, with this conventional arrangement, partly because the range of color reproduction is inadequate for light from the central areas of the cathode ray tube, and partly because light from the peripheral areas of the cathode ray tube is reflected up to the essential light spectrum, the display color would become blue-emphasized green.
In connection with the above-mentioned existing projection television set and the improved projection television proposed by the applicant herein, there are still several disadvantages in the process of forming the aforementioned multilayered interference coating layer on the inner surface of the cathode ray tube with respect to the characteristic of the coating layer.
Specifically, as it will be explained later, since the multilayered interference coating layer is composed of 10 to 20 vacuum evaporation film of oxide titanium or oxide silicon, it is not an easy operation to form the vacuum evaporation film on the inner surface of the cathode ray tube having a cylindrical structure with a bottom.
Further, as the matter of course, in the cathode ray tube, a consideration is necessary for the high level withstand characteristic against X ray and electron beams.
As another consideration, a highly strict requirement of a high chemical resistance is also necessary when a fluorescent material is interposed between the multilayered interference coating layer and the vacuum evaporation aluminum film.
Accordingly, this invention is made to solve the above-described drawbacks of the existing projection type television, and an object of the present invention is to provide a projection type television apparatus which is capable of improving the color purity at the central and the periphery of the green cathode ray tube, whether or not the cathode ray tube has a multilayered interference filter interposed between the face plate pane and the fluorescent material.